This invention relates to communication systems and more particularly to wireless communication systems.
The number of frequency bands available for communication in mobile communication systems continues to increase, as does the amount of time needed for a user equipment (UE), such as a mobile phone or other remote terminal, to search for cells and public land mobile networks (PLMNs). In addition, increased search time requires increased power consumption in usually power-limited UEs.
Mobile communication systems include time-division multiple access (TDMA) systems, such as cellular radio telephone systems that comply with the GSM telecommunication standard and its enhancements like GPRS/EDGE, and code-division multiple access (CDMA) systems, such as cellular radio telephone systems that comply with the IS-95, cdma2000, and wideband CDMA (WCDMA) telecommunication standards. Digital communication systems also include “blended” TDMA and CDMA systems, such as cellular radio telephone systems that comply with the universal mobile telecommunications system (UMTS) standard, which specifies a third generation (3G) mobile system being developed by the European Telecommunications Standards Institute within the International Telecommunication Union's IMT-2000 framework. The Third Generation Partnership Project (3GPP) promulgates the UMTS and WCDMA standards.
3G mobile communication systems based on WCDMA as the radio access technology (RAT) are being deployed all over the world. High-speed downlink packet access (HSDPA) is an evolution of WCDMA that provides higher bit rates by using higher order modulation, multiple spreading codes, and downlink-channel feedback information. Another evolution of WCDMA is Enhanced Uplink (EUL), or High-Speed Uplink Packet Access (HSUPA), that enables high-rate packet data to be sent in the reverse, or uplink, direction.
New RATs are being considered for evolved-3G and fourth generation (4G) communication systems, although the structure of and functions carried out in such systems will generally be similar to those of earlier systems. In particular, orthogonal frequency division multiplexing is under consideration for evolved-3G and 4G systems.
This application focusses on WCDMA and GSM radio access technologies for simplicity of explanation, but it will be understood that the principles described in this application can be implemented in communication systems employing other RATs.
Cell and PLMN selection has a number of objectives, which include connecting a UE to the cell(s) and PLMN(s) that will provide the highest quality of service (QoS), enable the UE to consume the least power, and/or generate the least interference. Cell/PLMN selection is usually based on the signal strength (signal to interference ratio (SIR) or signal to noise ratio (SNR)) of candidate cells. For example, U.S. patent application Ser. No. 11/289,001 filed on Nov. 29, 2005, by B. Lindoff for “Cell Selection in High-Speed Downlink Packet Access Communication Systems” describes a cell selection process that takes into account the delay spread of the communication channel. U.S. Patent Application Publication No. US 2002/0119774 for “Method for PLMN Selection” by Johannesson et al. describes how a UE receives a list of data associated with networks neighboring the PLMN currently serving the UE from a base station (BS) of the PLMN currently serving the UE. A new PLMN to serve the UE can be selected based upon the list. U.S. Patent Application Publication No. US 2004/0224689 for “Method for a Radiotelephone to Scan for Higher Priority Public Land Mobile Network” by Raghuram et al. describes how a radiotelephone can scan for available frequencies that are in use and supported by higher priority PLMNs and the radiotelephone.
For 3GPP-compliant mobile communication systems, the PLMN selection process is specified in Section 4.4 of 3GPP Technical Specification (TS) 23.122 V7.5.0, Non-Access-Stratum (NAS) functions related to Mobile Station (MS) in idle mode (Release 7) (June 2006). At switch on, or following recovery from lack of coverage, the UE typically searches for the registered PLMN (RPLMN) or equivalent PLMN (if it is available) using all RATs that the UE is capable of, one RAT after another. All frequencies in all bands belonging to each RAT are searched for a measured signal strength, e.g., a received signal strength indicator (RSSI), above a RAT-specific search level. The search is done to determine which frequencies carry physical channels and which do not. If successful registration is achieved, the UE indicates the selected PLMN. If there is no RPLMN, or if registration is not possible, the UE follows either an automatic or a manual specified selection procedure, depending on its operating mode.
Today, the same geographic area is often served by two or more different RATs, e.g., WCDMA and GSM. Only a subset of the frequency bands supported by a RAT is typically used in a given geographic area, and one or more of the frequency bands of different RATs may overlap in part or completely. Frequencies that are valid for multiple RATs can carry physical channels of only one RAT at a time at a given geographic location. For all other RATs, the energies on those frequencies will be identified as noise.
According to Section 5 of 3GPP TS 25.101 V7.4.0, User Equipment (UE) radio transmission and reception (FDD) (Release 7) (June 2006) and Section 2 of 3GPP TS 45.005 V7.6.0, Radio Access Network; Radio transmission and reception (Release 7) (June 2006), the center frequencies of GSM and WCDMA frequency channels are denoted as Absolute Radio Frequency Channel Number (ARFCN) and UTRA Absolute Radio Frequency Channel Number (UARFCN), respectively. UTRA is an abbreviation of UMTS Terrestrial Radio Access, and UMTS is an abbreviation of Universal Mobile Telecommunications System. The frequency channels are typically placed on 200 kHz channel raster, which is common for both systems although WCDMA channels are 5 MHz wide and GSM channels are 200 kHz wide.
In some current UE implementations, a RAT device searches all frequency bands supported by a first RAT, e.g., WCDMA, and then switches to a second RAT, e.g., GSM, and searches again if no cell is found in the first search. For example, there are 300 shared frequency channels (ARFCNs/UARFCNs) in a 60-MHz-wide, shared frequency band. A typical scan for GSM cells involves searching an entire downlink frequency band to detect energy on each 200 kHz-wide channel, which may be indicated by a respective signal strength or RSSI. If energy is detected on a channel, a further search is performed in order to identify cell(s). If no GSM cell is actually present, the UE may spend a significant amount of time before this condition is recognized. Scans for WCDMA cells and cells of other RATs that may share the frequency band typically follow the same principles.
Thus, searching all possible physical channels of all RATs that the UE is capable of in overlapping parts of the frequency spectrum increases the total search time and power consumption of the UE, with clear negative impact on both battery life time and user perceived experience.